In the technique of overhead rail conveyor systems, of the type for conveying parts along assembly lines, the problems caused by the need to move along sloping sections are well known. In order to prevent any slippage of the driving wheel from leading to erratic or impossible movement on slopes, various solutions have been put forward, the most widely used being those of exerting intense preloading of the wheel by means of pressure rollers acting on the opposite side of the rail, and using racks and toothed wheels. The first solution gives rise to intense friction, which is unjustified when travelling on level sections, and is substantially inefficient for conveying heavy loads, or for steep gradients. The second solution ensures a driving thrust even for the heaviest loads, but presents serious problems at the beginning of upward sloping ramps. In fact, in order to cut costs, the racks are provided only on the sloping sections. Upon reaching the beginning of one of these sections, the toothed wheel therefore has great difficulty in meshing, resulting in wear and considerable noise. The general scope of this invention is to obviate the aforementioned problems by providing an overhead rail conveyor system in which the sloping sections can be easily traversed even with very heavy loads or steep gradients, without problems of fit and with relatively low costs.